[Crito]

Server-Server and Servier-Client SSL is a thing for IRC. Of course if you operate one of the servers then you naturally see everything that goes through it. Any anybody in the same channel sees everything in that channel, since that is the point of IRC.

IRC clients typically also support DCC, though I am unaware of what the encryption options there are. There are are other forms of encrypted "IMing" however, if you want secure peer-to-peer text chat you should probably look outside what irssi has to offer.